1. Field of the Invention
The present invention relates generally to communication systems that use Time Division Multiple Access (TDMA) techniques.
2. Description of Related Art
Wireless access in a land mobile radio system is provided by base stations (BSs) located at well-chosen spots. The base stations are equipped with antenna arrangements which give radio coverage in a specific geographical area. These coverage areas are often denoted cells, and, as a consequence, the mobile radio systems are often referred to as cellular radio systems.
Land mobile radio systems are designed to reuse frequency channels. The frequency reuse improves the system""s spectrum utilization by allowing the same frequency to be reused in different geographical locations. However, the frequency reuse also implies cochannel interference which is a disadvantage because it degrades the quality of the transmission. A measure of the frequency reuse is the cluster size. A cell cluster is defined as the minimum amount of cells which contain all available frequency channels. The cluster size is a measure of the area coverage. The spectrum efficiency xcex7 is an indication of merit for a cellular system. Several different definitions of spectrum efficiency exist, but most of them can be expressed as                     η        =                  Γ                      W            ·            β            ·            A                                              (        1        )            
where
xcex7=spectrum efficiency,
xcex93=throughput per link,
W=bandwidth (in Hertz) per link,
xcex2=cluster size,
A=area of a cell.
A common assumption is to keep the cell-area, A, constant, and account for the effect of frequency reuse by changing xcex2. The bandwidth W is typically a system-specific (given) parameter. What remains to be improved are xcex93 and the cluster size xcex2. The improvements proposed here will improve one or both of xcex93 and xcex2 by allowing an increased xcex93, a smaller xcex2, or both.
In a communications system such as a wireless communications system having base stations and mobile stations organized into cells and employing TDMA techniques, throughput and capacity in the system are increased by simultaneously transmitting data over a plurality of different multipath or radio channels using the same carrier frequency.
In accordance with a first exemplary embodiment of the invention, information sequences are transmitted simultaneously via different uncorrelated radio-paths from a transmitter, for example a base station, and are each intended for different receivers, for example mobile stations. Each receiver uses known synchronization information to subtract undesired information sequences from the signal it receives, so that only the desired information sequence remains. This technique increases the total spectrum efficiency in the TDMA system by allowing a smaller cluster size.
In accordance with a second exemplary embodiment of the invention, different sequences are transmitted simultaneously via different uncorrelated radio paths, and all of these sequences are intended for the same receiver. The receiver uses known synchronization information to separate the different information sequences, thus providing the receiver with a higher throughput in bits/second.
In accordance with a third exemplary embodiment of the invention, a transmitter transmits identical information sequences over reasonably uncorrelated radio paths to a receiver. A space diversity effect results in the receiver, and the result is that the number of channels per area-unit can be increased using diversity gain. This enables an increase in traffic, and thus an increased spectrum efficiency.
In accordance with exemplary embodiments of the invention, a system can dynamically employ one or more of the above techniques to trade user throughput for number of users, or vice versa, in response to changing demands. For example, a wireless communications service provider can choose in some cases to provide a high throughput to a small number of users, while in other cases it can choose to support a large number of users in a given area.
In accordance with exemplary embodiments of the invention, the transmitter can be a base station in a wireless communications network or system, and the receivers can be mobile stations in the wireless communications network.